Tunable resonant cavity



July 17, 1962 R. w. HAEGELE ETAL 3,045,146

TUNABLE RESONANT CAVITY Filed March 18, 1959 ATTO'QNEKS United StateslPatent O 3,045,146 TUNABLE RESONANT CAVITY Rowland W. Haegele, Salt LakeCity, Utah (Menlo Park, Calif.), and John W. McLaughlin, Salt Lake City,Utah (San Jose, Calif.), assignors to Etel-Mc- Cullough, Inc., SanBruno, Calif., a corporation of California Filed Mar. 18, 1959, Ser. No.800,326 10 Claims. (Cl. S15- 5.21)

This invention relates to a tunable resonant cavi-ty, and, moreparticularly, to a tunable resonant cavity in relation with an electrontube.

One particular type of electron tube which has a rtunable cavity is areflex klystron. A reflex klystron cornprises an electron gun, a tunableresonant cavity `and a reector electrode. The electron gun generates abeam of electrons which is projected across an interaction gap fandthrough the resonant cavity toward the reijlector electrode. In crossingthe interaction gap, electromagnetic oscillation in the resonant cavitywhich surrounds the gap will tend to accelerate certain electrons of thebeam and decelerate others. The electrons will proceed on toward thereflector electrode Where they will be reflected back through the gapand during the course of which the lfast electrons will tend to catch upwith the slow electrons to lform bunches. The bunches of elecwhen readin conjunction with the accompanying singleV trons will tend to increasethe electromagnetic oscillations in the resonant cavity when theyreoross the interaction gap. Thus, the electromagnetic oscillations willbe maintained and power may be coupled from the resonant cavity.

There are many methods `for tuning the cavity, and one practical methodis to adjustably insert a metallic plunger or post into the cavitythrough a wall of the cavity by means of engaging screw threads on `theplungers land cavity wall. However, whenever metal-to-metal contact ispresent in a cavity such as occurs ybetween the post and cavity wallthreads, relative movement of the metal parts will cause noise or staticin the output of the tube. The relative movement of the metal parts willvary the electrical resistance at the interface therebetween, and sincethe current across the interface is high, the power (12R) losses acrossthe interface will also vary. The noise or `static thus produced wil-lupset the proper operation of an electronic system which uses the outputof the klystron, particularly during tuning of the k-lystron.

According to this invention, the problem of noise and stat-ic duringtuning is. eliminated through the use of a non-contacting tuner. In thenon-contacting tuner Iaccording to this invention, there is nomechanical contact between the tuning plunger-and the cavity wall, butan adequate high frequency (hereinafter referred to as HF.) short isprovided at the usual contact point by means of a shortedhalf-wavelength line, and the metal-tometal contact between the threadsof the tuning post and cavity Wall -is placed along such line at yapoint where the electrical current is approximately zero.

- It is an object of this invention to provide a tunable resonant cavityin which tuning noises and static are reduced.

It is another object of the present invention to provide a tunableresonant cavity with non-contacting H.F. short between the cavity wallsur-face and the tuning plunger.

It isa further object of the present invention to provide a tunableresonant cavity with the sliding metal-to-metlal contact of the tuningcontrol means of the tunable cavity where the electrical current isapproximately zero.

It is yet another object of this invention to provide a reflex klystronin which tuning noise and static are reduced.

ful

ICC

These and other objects of the invention will become more clearlyapparent from the following description gure drawing which is asectional View of a reilex klystron taken through the klystron axis, andthrough the tuning cavity.

Referring now to the drawing, there is shown a reflex klystron with atubular body l2, a cathode assembly 14, a repeller assembly 16, `and atuning assembly 18.

rihe cathode assembly i4 is composed of a series of stacked ceramicrings with metallic rings disposed interstitially between the ceram-icrings. A dish-shaped disk forms a vacuum wall of the klystron tube atthe cathode end. The disk 20 has an outwardly protruding ange 21, whichis brazed between two ceramic cylinders 22 and 24. The ceramic ring 24forms a part of the vacuum envelope, and the ceramic ring 22 is abacking ring which reinforces the metal-to-ceramic braze. On ythe otherend of the ceramic ring 24 is lbrazed an outwardly protruding flange 25formed on the large end of a truncated cathode support cone 26. Thesmaller end of the truncated cone is formed into a cylinder 28, whichsupports a disk cathode with the aid of a thin cylindrical heat shield32. A stepped-sided cylindrical fo cusing electrode 34 is fixed by itssmaller diameter end also to cylinder Z8. A helical filament 36,disposed within the cathode 30 and heat shield 32, heats the cathode.Two leads 3S and 40 xed to the disk 20 support cone 26, respectively,conduct electricity to the tilament 36. A heat conducting cylinder 41,disposed `around the lilament, is welded by an outward protruding angeto the underside of cathode 30 to transfer heat from the filament 36 tothe cathode 30.

The cathode assembly is completed by brazing a ceramic ring 42, whichforms a part of the vacuum enceramic disks with metallic rings disposedinterstitially Y between them. The end vacuum wal-l of the repellerassembly is an inwardly disposed truncated cone 48 which supports anexhaust tubulation 50. The conehas an outwardly extending tlange 51 onits larger end onto which is brazed a ceramic backing ring 52. On theother end of the ange 51 is brazed an outwardly extending ange ofrepeller support cylinder S4, and to the iiange of the support cylinder54 is brazed ceramic ring 56, which is la part of the vacuum envelope. Asealing ring 58 is -brazed to the opposite end of ceramic ring 56. Aceramic backing ring 60 is brazed to the opposite side of the sealingring 58, reinforcing the metal-toceram'ic braze. A repeller 62. isbrazed on the free end of the repeller support cylinder 54.

The body l2. of the reflex klystron is in a tubular form. Within thebody 12 are located a drift `tube anode 64 and an annular metal plate 66which form part of the resonant cavity of the klystron. At the end ofthe drift tube anode 64 adjacent the cathode is disposed an anodeaccelerating grid 6e, and on the other end of the drift tube is disposeda gap grid 70. A second gap grid 72 is disposed in the opening of theannular plate 66. The drift tube yanode 64, together with the annularplate 66, grids '70 and 72, form a primlary cavity 74 which is disposedinternally of the vacuum envelope of the reflex klystron. The primarycavity 74 has an aperture 76 which leads into anl external secondarycavity 77 which is -disposed externally of the vacuum envelope.Caviti'es 74 yand '717 are electrically coupled by apertures 76 so thatthe tuning of the secondary cavity 77 will control l the frequency ofthe primary cavity 74 and have characteristics of a single cavity.

The secondary cavity 77 is formed by rectangular metal tube 78, which isbrazed into a rectangular port 80 in the side of the body 12. The tube78 is disposed perpendicularly to the axis of the body 12. The internalend of the rectangular tube 78 is closed with a metal plate 82, intowhich the aperture 76 is formed. A ceramic window 84 brazed over theopening 76 makes a vacuum-tight seal. The other end of the rectangulartube 78 is closed by metal end plate 86, which has an output aperture88, and a waveguide connecting flange 90 is welded externally of therectangular tube 78 for coupling waveguides to the cavity. A matchingplug 91 is placed in the secondary cavity wall to control the couplingbetween the primary cavity 74 and the secondary cavity 77, and maintaina proper level of coupling over the tunable range of the klystron.

Through one wall of the rectangular tube 78 and parallel to the axis ofthe klystron is disposed a tuning plunger assembly 92.

The repeller assembly 16 and the cathode 14 are mounted to opposite endsof body 12 with the aid of sealing rings 94 and 96 brazed to the ends ofthe body 12. The vacuum seal is made by arc-welding the mating ends of asealing ring 94 to the sealing ring 58. The tube is evacuated through atubulation 50 which is pinched olf to seal the tube. The backing rings60 and 46 prevent the repeller and cathode from squeezing into the body12 and misaligning the electrodes and also allow for uneven thermalexpansion between the metallic body and the ceramic insulators. Thecathode assembly and repeller assembly are coated externally with aninsulating plastic 98 and 100.

In operation, electrons are emitted from the heated cathode 30 focusedby the focusing electrode 34 towards the klystron axis, and acceleratedby the anode grid 68. The electrons pass across the interaction gapformed by grids 70 and 72. On passing across the gap, some electrons areaccelerated and other electrons are decelerated, forcing the electronsto bunch axially. The repeller refleets the electrons back across thegap. The relative voltage and space between the cathode, anode andrepeller are so that the electrons are effectively bunched as theyrecross the gap and add power to the high frequency oscillations in theresonant cavity.

The operating frequency of the reex klystron is controlled by the tuningplunger assembly 92. The frequency decreases when a metall tuningplunger 102 is screwed into the secondary cavity 77. The tuning plunger102 is disposed parallel to the tube axis. The plunger 102 is threadedinto internal threads of a tubular tuning plunger support ,104, which inturn is held at one end by a short bushing 106 that is brazed into abore formed in the rectangular tube 78. The internal threads of theplunger support 104 which engage the plunger are conned to its free end.The end of support 104 adjacent the cavity has a larger internaldiameter than the other end. A sleeve 108 is brazed internally of thebushing 106 and a braze 110, which is located at the ends of sleeve 108and bushing 106 adjacent the cavity. The outer diameter of sleeve 108 is-smaller than the inner diameter of support 104 and the bore of bushing106 into which the bore of -the sleeve is disposed, forming acylindrical void 112. The free end of sleeve 108 is spaced from theinternal threads on support 104 so as to provide a radial void 114 whichcommunicates with void 112 and also communicates with the space betweenthe plunger 102 and sleeve 108. Voids 112 and 114 are one-quarterwavelength long at the average operating frequency of the tube. Thedistance from the cavity surface to void 114 is also aquarter-wavelength long at the operating frequency. Together they form ashorted half-wavelength line.

As is well known in the art, the folded void formed by voids 112 and 114forms a shorted quarter-wavelength choke. Accordingly, since the foldedvoid is shorted at one end, the voltage at that end is zero and thecurrent is at its highest value. Therefore, at a point one-quarterwavelength from the short the voltage is at its highest and the currentis zero. Since the current at this point is Zero, the thread contactsbetween the post 104 and support 106 was placed as close to this pointas practical. As stated previously, the tuning noise is caused by thevarying resistance across the sliding thread contact and thereforecausing a varying power loss. The electrical resistance across thesliding thread contacts cannot be controlled, but the power losses canbe controlled by providing a quarter-wavelength choke. The power loss iscomputed by the well-known formula P=I2R. Therefore, if the I (current)is zero, the power loss is always zero independent of the value of R(resistance).

Since the distance from the cavity surface to the void 114 is alsoone-quarter wavelength, an H.F. short (zero voltage) appears at thecavity surface. The same properties of a high frequency alternatingcurrent are reproduced every half-wavelength. The H.F. path from theshorted end of void 112 to the cavity is a half-wavelength, therebyreflecting the short to that point. In keeping with the spirit of theinvention, the metal-to-metal con tact can be placed at any oddmultiples of quarter-wavelength from the short, and the cavity surfacecan be placed on multiples of half-wavelength from the short.

According to an alternative embodiment, the folded quarter-wavelengthchoke may be placed in the tuning plunger 102. The circular opening ofthe choke will still be adjacent the threads on the post so that themetallic sliding contact will be in a region of low current. Accordingto this embodiment the choke will also communicate with the cavitythrough the space between the plunger 102 and sleeve 108, thus providinga H.F. short at the point where the plunger enters the cavity.

We claim:

l. An electron tube comprising a vacuum envelope, a plurality ofelectrodes within said envelope, a tunable resonant cavity comprising aconductive wall means forming said cavity, and an interaction gap forproducing electromagnetic oscillation therein, said wall means having abore therethrough communicating with said cavity, a tuning means forsaid cavity comprising a conductive tuning plunger extending throughsaid bore in spaced relation to the wall thereof, means including saidwall means and said plunger forming a shorted half-wavelength coaxialtransmission line electrically terminating in said cavity, frictionmeans for retaining said plunger in fixed relation with said cavity andelectrically connected to said wall means making sliding conductivecontact with said plunger near the quarter-wave point of said coaxialline.

2. An electron tube comprising a vacuum envelope, a plurality ofelectrodes within said envelope, a tunable resonant cavity comprising aconductive wall means forming said cavity, and an interaction gap forproducing electromagnetic oscillation therein, said wall means having abore therethrough communicating with said cavity, a tuning means forsaid cavity comprising a conductive tuning plunger extending throughsaid bore in spaced relation to the said wall means, means includingsaid wall means and said plunger forming a shorted half wavelengthcoaxial transmission line electrically terminating in said cavity,friction means for retaining said plunger in fixed relation with saidcavity and electrically connected to said Wall means making slidingconductive contact with said plunger at the quarter-wave point of saidcoaxial line whereby said sliding contact is disposed in a region ofminimum current.

3. An electron tube comprising a vacuum envelope, a plurality ofelectrodes within said envelope, a tunable resonant cavity comprising aconductive wall means forming said cavity, and an interaction gap forproducing electromagnetic oscillation therein, said wall means having abore therethrough communicating with said cavity, a tuning means forsaid cavity comprising a conductive tuning plunger extending throughsaid bore in spaced relation to the wall thereof, means including saidwall means and said plunger forming a folded shorted half-wavelengthcoaxial transmission line electrically terminating in said cavity, saidfold of said shorted half-wavelength coaxial transmission line disposednear the center thereof, Said plunger having external threads andengaging internal threads in said bore near said fold and outside thecoaxial transmission line whereby said contact between said threads isdisposed at the region of lower current and higher voltage.

4. An electron tube as set forth in claim 3 wherein said shortedtransmission line has its shorted end folded into said Wall means, andsaid internal threads of said bore extend to said fold in saidclosed-ended half-wavelength H.F. short.

5. An electron tube as set forth in claim 3 wherein said shortedtransmission line has its shorted end folded radially into said borescylindrical surface, and said internal threads of said -bore extend tosaid fold in said closedended half-wavelength short.

6. A reflex klystron tube comprising a vacuum envelope; a cathode, ananode, and a repeller electrode disposed coaxially within said envelope,a tunable resonant cavity having a portion disposed interiorly of saidenvelope and a portion disposed exteriorly of said envelope, saidenvelope having a high frequency aperture communicating between theinterior and exterior portions of said tunable resonant cavity, aceramic window covering said aperture and vacuum sealed to the peripheryof said aperture, said tunable resonant cavity comprising a conductivewall means forming said cavity, said wall means having a bore there'through communicating with said cavity, a tuning means for said cavitycomprising a conductive tuning plunger extending through said bore inspaced relation to the wall thereof, means including said wall means andsaid plunger forming a shorted half-wavelength coaxial transmission lineelectrically terminating in said cavity, friction means for retainingsaid plunger in ixed relation with said cavity and electricallyconnected to said wall means making sliding conductive contact with saidplunger near the quarter-wave point of said coaxial line.

7. A reflex klystron tube comprising a vacuum envelope; a cathode, ananode, and a repeller electrode disposed coaxially within said envelope,a tunable resonant cavity having a portionl disposed interiorly of saidenvelope and a portion disposed exteriorly of said envelope, saidenvelope having a high frequency aperture communicating between theinterior and exterior Portions of said tunable resonant cavity, aceramic window covering said aperture and vacuum sealed to the peripheryof said aperture, said tunable resonant cavity comprising a conductivewall means forming said cavity, said wall means having a boretherethrough communicating with said cavity, a tuning means for saidcavity comprising a conlin said cavity, friction means for retainingsaid plunger in lixed relation with said cavity and electricallyconnected to said wall means making sliding conductive contact with saidplunger at the quarter-wave point of said coaxial line whereby saidsliding contact is disposed in a region of minimum current.

8. A reflex klystron tube comprising a vacuum envelope; a cathode, ananode, and a repeller electrode disposed coaxially within said envelope,a tunable resonant cavity having a portion disposed interiorly of saidenvelope and a portion disposed exteriorly of said envelope, saidenvelope having a high frequency aperture communicating between saidinterior and exterior portions of said tunable resonant cavity, aceramic window covering said aperture and vacuum sealed to the peripheryof said aperture, said tunable resonant cavity comprising a conductivewall means forming said cavity, said wall means having a boretherethrough communicating with said cavity, a tuning means for saidcavity comprising a conductive tuning plunger extending through saidbore in spaced relation to the wall thereof, means including said wallmeans and said plunger forming a folded shorted half-wavelength coaxialtransmission line electrically terminating in said cavity, said fold ofsaid shorted half-wavelength coaxial transmission line disposed near thecenter thereof, said plunger having external threads and engaginginternal threads in said bore near said fold and outside the coaxialtransmission line whereby said contact between said threads is disposedat the region of lower current and higher voltage.

9. A reflex klystron as set forth in claim 8 wherein said shortedtransmission line has its shorted end folded into said wall means, andsaid internal threads of said bore extend to said fold in saidclosed-ended half-Wavelength H.F. short.

10. A reflex klystron as set forth in claim 8 wherein said shortedtransmission line has its shorted closed end folded radially into-saidbores cylindrical surface and said internal threads of said bore extendto said fold in Said closed-ended half-wavelength H.F. short.

References Cited in the le of this patent UNITED STATES PATENTS

